If you are using Grails Web Flow and put domain object in flash scope.

flash.model = new MyModel()

Make sure your events beforeInsert, beforeUpdate, beforeDelete, etc. are marked transient. Otherwise the domain object cannot be serialized and you get the following exception:

Could not serialize flow execution; make sure all objects stored in flow or flash scope are serializable; nested exception is java.io.NotSerializableException
 

This happens because closures cannot be serialized.

Always mark your closues as transient to avoid future serialization problems.

class MyModel implements java.io.Serializable {

transient def beforeInsert = { … }

}